1. Field of the Invention
The present invention relates to a pulse-width modulated (PWM) rectifier for converting a three-phase alternating current into a direct current by using a semiconductor device whose switching operation is controlled by a PWM signal.
2. Description of the Related Art
In a motor control apparatus that drives a machine tool, industrial machine, robot, or the like, a converter (rectifier) is used to convert commercial power into DC power then supplies the DC power to an inverter that drives the motor.
In recent years, rectifiers using pulse-width modulation (PWM) are being widely used because of the need to reduce power supply harmonics and reactive power. In a PWM rectifier, switching losses occur because high-speed switching is performed using a semiconductor device. Accordingly, this type of rectifier has the problem that, compared with conventional rectifiers based on diodes, losses in the power converter increase and the size of the converter thus increases.
To solve this problem, the prior art has employed a method that reduces the PWM frequency in regions where the amplitude of the current is large. This method is effective in reducing losses in the power converter and suppressing the increase in the converter size.
However, the prior art method has had the disadvantage that the response of the control system degrades because the feedback sampling period becomes longer as the PWM frequency decreases.
JP9-252581A discloses a method in which the carrier frequency of the rectifier (PWM converter) is varied. Further, JP2004-48885A and JP63-290170A each disclose a power converter that produces power from DC voltage by pulse modulation and supplies the power to a load, such as an electric motor, with provisions made to switch the modulation scheme between a three-phase modulation scheme and a two-phase modulation scheme (more properly, a modified two-phase modulation scheme: Refer to “PWM Power Conversion System” by Katsunori Taniguchi, Kyoritsu Publishing Co., Ltd, PP. 96-98). JP2008-259343A discloses a converter-inverter constructed by connecting an inverter to a converter, with provisions made to employ the modified two-phase modulation scheme as the PWM modulation scheme for either the converter or the inverter.